The present invention relates to a cathode ray tube and more particularly to a color cathode ray tube including an electron gun having an electron lens by which the focus characteristic in a small beam current region is improved. A cathode ray tube which is used for color image display and color monitor (hereinafter referred to as a color cathode ray tube) includes a vacuum envelope comprising a panel portion which is a display screen, a neck portion for housing an electron gun and a funnel portion for connecting the panel portion and neck portion. In the funnel portion, a deflection device for scanning an electron beam emitted from the electron gun on the phosphor screen coated on the inner surface of the panel is mounted.
The electron gun housed in the neck portion comprises various electrodes such as a cathode, a control grid, a focus electrode, and an accelerating electrode, modulates an electron beam from the cathode by a signal applied on the control grid, shapes it into one having a required sectional shape and energizes it via the focus electrode and the accelerating electrode, and makes it impinge onto the phosphor screen.
The electron beam is deflected in two horizontal and vertical directions by the deflection device installed around the funnel portion on the way from the electron gun to the phosphor screen and forms an image on the phosphor screen.
As a model of this kind of electron gun, for example, Japanese Patent Application Laid-Open SHO 53-51958 discloses an electron gun comprising a first accelerating electrode, a focus electrode, and a second accelerating electrode toward the phosphor screen in the order named.
For example, FIGS. 17 and 18 are drawings for comparison of structures of two electron guns in terms of focus voltage application scheme and they are axial cross sectional views of in-line type electron guns viewed in a direction of the in-line arrangement. FIG. 17 shows a fixed focus voltage type and FIG. 18 shows a dynamic focus voltage type.
In FIGS. 17 and 18, a numeral 01 indicates a first electrode means for generating an electron beam and directing the electron beam toward the phosphor screen, 02 a second electrode means which constitutes a main lens for focusing the electron beam onto the phosphor screen, 03 a cathode, 04 the first grid, 05 the second grid, 06 a first accelerating electrode (the third grid), 07 a focus electrode (the fourth grid), 07-1 a first member of the focus electrode, 07-2 a second member of the focus electrode, 07-3 an electrode plate, 08 a second accelerating electrode (the fifth grid), 08-1 an electrode plate, and 09 a shield cup.
In FIG. 18, a numeral 07-4 indicates an electrode plate and 07-5 indicates a correction electrode plate.
In FIG. 17, the first electrode means 01 comprises the cathode 3, the first grid 04, and the second grid 05 and the second electrode means 02 comprises the first accelerating electrode 06, the first member of focus electrode 07-1, the second member of focus electrode 07-2, the electrode plate 07-3, the second accelerating electrode 08, and the electrode plate 08-1.
In FIG. 18, the first electrode means 01 comprises the cathode 3, the first grid 04, and the second grid 05 and the second electrode means 02 comprises the first accelerating electrode 06, the first member of focus electrode 07-1, the second member of focus electrode 07-2, the electrode plate 07-3, the electrode plate 07-4, the correction electrode plate 07-5, the second accelerating electrode 08, and the electrode plate 08-1.
A symbol d.sub.4 indicates the diameter of the electron beam passage aperture of the second grid 05 on the side of the first accelerating electrode 06, d.sub.1 the diameter of the electron beam passage aperture of the first accelerating electrode 06 on the side of the second grid 05, d.sub.5 the diameter of the electron beam passage aperture of the first accelerating electrode 06 on the side of the first member of focus electrode 07-1, D the diameter of the aperture of the main lens, L.sub.1 the length of the first accelerating electrode 06, d.sub.2 the spacing between the first accelerating electrode 06 and the first member of focus electrode 07-1, L.sub.2 the length of the first member of focus electrode 07-1, d.sub.3 the spacing between the first member of focus electrode 07-1 and the second member of focus electrode 07-2, L3 the length of the second member of focus electrode 07-2, L the sum of the length L.sub.2 of the first member of focus electrode 07-1, the length L.sub.3 of the second member of focus electrode 07-2, and the spacing d.sub.3 therebetween, L.sub.4 the sum of the length L.sub.1 of the first accelerating electrode 06, the length L.sub.2 of the first member of focus electrode 07-1, the spacing d.sub.2 therebetween, the length L.sub.3 of the second member of focus electrode 07-2, and the spacing d.sub.3 between the first member of focus electrode 07-1 and the second member of focus electrode 07-2, V.sub.f a focus voltage, Eb an accelerating voltage, and Vd a voltage which changes in synchronization with deflection of an electron beam.
In the electron gun of the aforementioned constitution, the sum L of the length L.sub.2 of the first member of focus electrode 07-1, the length L.sub.3 of the second member of focus electrode 07-2, and the spacing d3 therebetween exceeds 1.1 times the diameter D of the aperture of the main lens and the sum L.sub.4 of the length L.sub.1 of the first accelerating electrode 06, the length L.sub.2 of the first member of focus electrode 07-1, the spacing d.sub.2 therebetween the length L.sub.3 of the second member of focus electrode 07-2, and the spacing d.sub.3 between the first member of focus electrode 07-1 and the second member of focus electrode 07-2 is within the range from 4 to 5.4 times the diameter D of the aperture of the main lens.
The diameter d.sub.4 of the electron beam passage aperture of the second grid 05 on the side of the first accelerating electrode 06 and the diameter d.sub.1 of the electron beam passage aperture of the first accelerating electrode 06 on the side of the second grid 05 are very small compared with the diameter D of the aperture of the main lens.
It is well known that main factors which determine the electron beam spot diameter (hereinafter, it may be referred to as just a beam spot diameter) are space charge effect, thermal initial velocity spread, and spherical aberration of a main lens.
The maximum diameter of an electron beam spread in the main lens traveling from the cathode toward the phosphor screen (hereinafter, it may be referred to as just a beam diameter in the main lens) is related with the beam spot diameters determined by the two aforementioned factors respectively as described below. When the beam diameter in the main lens is denoted in the abscissa and the beam spot diameter is denoted in the ordinate, the beam spot diameter determined by the spherical aberration of the main lens draws a right upward curve which increases as the beam diameter in the main lens increases and the beam spot diameter determined by the space charge effect and thermal initial velocity spread draws a right downward curve which decreases as the beam diameter in the main lens increases.
The relationship between the beam diameter in the main lens and the beam spot diameter determined by the two aforementioned factors is obtained by combining the beam spot diameters determined by the two aforementioned factors respectively and indicated by a quadratic-like curve which initially decreases and then increases as the beam diameter in the main lens increases. Therefore, there exists an optimum beam diameter in the main lens which minimizes the beam spot diameter determined by the two aforementioned factors. The beam diameter in the main lens which minimizes the beam spot diameter determined by the two aforementioned factors varies with the current emitted from the cathode. For an electron gun of a color cathode ray tube, the length of each electrode is optimized so that the beam diameter in the main lens minimizes of nearly minimizes the beam spot diameter determined by the two aforementioned factors in a large beam current region.
In an electron gun of the aforementioned constitution, the accelerating voltage Eb which is the highest voltage is applied to the first accelerating electrode, so that an electron lens having a very strong focusing action is formed between the first electrode means and the first accelerating electrode. Therefore, a small crossover can be formed even in the large beam current region. The electron beam in the main lens after forming crossover spreads nearly to the beam diameter in the main lens which minimizes the beam spot diameter determined by the two aforementioned factors, so that the beam spot diameter in the large beam current region can be decreased.
When the length L of the focus electrode is made longer than 1.1 times the diameter D of the aperture of the main lens or the resultant increase in the focus voltage becomes 24% or more of the accelerating voltage, the spherical aberration of the main lens can be decreased. The beam spot diameter can be decreased also in this respect.